Total knee arthroplasty involves the replacement of portions of the patella, femur, and tibia with artificial components. In particular, a proximal portion of the tibia and a distal portion of the femur are cut away (resected) and replaced with artificial components.
The femoral component is generally a metallic alloy construction (e.g. cobalt-chrome alloy or 6A14V titanium alloy) and provides medial and lateral condylar bearing surfaces of multi-radius design of similar shape and geometry as the natural distal femur or femoral-side of the knee joint.
FIG. 1A is an example of a standard femoral component 7. The interior of the component has five planar fixation surfaces 1-5 and may also include an intramedullary (“IM”) stem 6. Prior to installation of the component 7, the distal femur must be prepared so that it has five fixation surfaces which match the interior surfaces of the component. More particularly, and with reference to FIG. 1B, a distal femur 8 must be resected to have a distal cut surface 1′ (corresponding to surface 1 in FIG. 1A), a posterior cut surface 2′ (corresponding to surface 2 in FIG. 1A), an anterior cut surface 3′ (corresponding to surface 3 in FIG. 1A), an anterior chamfer cut surface 4′ (corresponding to surface 4 in FIG. 1A) and a posterior chamfer cut surface 5′ (corresponding to surface 5 in FIG. 1A). These cuts are typically made with oscillating saw blades. In scenarios where additional stability is desired, the IM canal of the distal femur 8 may be reamed to accept an IM stem 6 of a femoral component 7. A femoral component without an IM stem may also be used.
A femoral component may be located with six degrees of freedom relative to the patient's femoral geometry. These may be expressed in a Cartesian manner relative to orthogonal anatomical reference planes as shown in FIG. 2. There are three types of angulation/rotation, namely varus-valgus angulation, flexion-extension angulation, and internal-external rotation. Additionally, there are three types of linear position, namely inferior-superior, anterior-posterior and medial-lateral. It should be noted that the joint shown in FIG. 2 is rotated into flexion. As a result, the anterior and posterior sides of the distal femur are not arranged along the antero-posterior position axis. To position a distal femoral component on the bone, a number of datum features of a patient's anatomy and their relative location as controlled by soft tissue structures at the knee may be utilized.
A goal of total knee arthroplasty is to resect the damaged bone on the distal femur and replace it with the same amount (e.g., volume) of material (e.g. metal of the distal femoral component implant) as the normal undamaged bone that was there prior to damage, thus restoring normal ligament tension and kinematics (motion) to the repaired joint. Current and historical instrumentation techniques resect the distal femur perpendicular to the femoral mechanical and longitudinal (anatomic) axis in the sagittal plane. FIG. 1B shows the typical 90 degree angle of the distal cut surface 1′ with respect to the longitudinal axis 9. Conventionally, the distal cut is the first resection to be made, the remaining four cuts (i.e. anterior, posterior, anterior chamfer, and posterior chamfer) typically being made with respect to the distal cut surface 1′ after it has been prepared.
In ideal circumstances, a femoral component exists with a femoral component size which exactly matches the specific femur size for any given patient. FIG. 3A shows an ideally sized femoral component 30 fitted on a distal femur. In this scenario, the posterior flange 12 of the femoral component replaces the posterior resected bone with the same amount of metal and the anterior flange 13 of the femoral component is flush with the anterior femur.
An issue arises when no femoral component size exactly matches the distal femur size of a specific patient. Femoral components are traditionally made in “stock” or standardized sizes. The “ideal” fit illustrated in FIG. 3A is generally not attainable with stock femoral component sizes which are not optimized for specific patients. A finite number of sizes are produced by the manufacturer and a specific patient is fitted with whichever available stock femoral component size is closest to the unique distal femur size of that specific patient. As a rough analogy, comparison may be made to pant sizes at a department store, where a limited number of sizes are offered according to standardized waist and inseam measures. A “perfect” fitting pair of pants would be made from raw textiles which are measured, cut, and sewn by a tailor for a specific individual. Producing custom femoral components in knee arthroplasty is cost prohibitive, and thus the practice of selecting the closest fitting femoral component size from a finite selection of sizes remains the industry standard.
There are two approaches on how to accommodate the issue of an inexact match between the sizes of available stock femoral components and the unique distal femur size of a specific patient.
The first approach is called “anterior referencing”, where the anterior cut for the femoral component is made at the normal position in order to avoid notching the femoral bone on the anterior side and potentially lead to future femoral fracture. In reference to FIG. 1B, anterior referencing provides anterior cut surface 3′ such that the amount of anterior bone resected substantially matches the volume of the anterior flange 13 of the femoral component 7. In order to permit the knee to rotate into flexion if the size of the patient-specific femur is between the standard femoral component sizes, the surgeon undersizes the femoral component. As an illustrative example, consider a scenario where there is a standard femoral component size “M” and a standard femoral component size “S”, where “S” is the closest available size to “M” that is also smaller than “M”. If “M” is larger than the patient-specific femoral size and “S” is smaller than the patient-specific femoral size, the surgeon selects the femoral component of size “S” for implanting in the specific patient in question. Undersizing in this manner requires cutting more posterior bone for posterior cut surface 2′ (FIG. 1B) than is replaced with femoral component material (e.g., metal). This causes an abnormal looseness of ligaments in flexion and possible clinical issues or implant longevity issues.
The other approach is called “posterior referencing”. In this case, the amount of posterior bone resected substantially matches the volume of the posterior flange 12 (FIG. 1A) of the femoral component 7 so there is no compromise in the ligament tension when in the knee is in flexion, thereby restoring the ligament balance to more normal conditions. If the patient-specific femur is between the stock femoral component sizes, the common practice is upsizing in order to prevent notching the anterior femur which can potentially cause fracture of the bone post operatively. However, upsizing causes an overstuffing of the patellofemoral space and can lead to limited motion, fracture of the patellar bone, or failure of the patella implant.
In order to minimize the potential operative and post-operative issues with each of the above two approaches, manufacturers have added additional stock sizes of femoral components thereby reducing the difference between stock femoral component sizes. This practice adds cost to the procedure and the industry in general.
FIGS. 3B and 3C show, respectively, an undersized femoral component and an oversized femoral component in traditional alignment, e.g., the distal resection has been made 90 degrees with respect to the longitudinal axis 9 of the femur. In other words, distal cut surface 1′ is orthogonal to the longitudinal axis 9 (see FIG. 1B). As shown, the undersized femoral component 31 and the oversized femoral component 32 are arranged with posterior referencing. FIG. 3B shows that to accommodate an undersized femoral component 31, the end of the anterior cut surface 3′ notches the anterior femur. In the alternative that the undersized femoral component 31 is moved to an anterior referencing position (still according to a traditional orthogonal alignment), more bone would be resected posteriorly than would be replaced with the femoral component's posterior flange. FIG. 3C shows how an oversized femoral component 32 may cause the anterior flange 13 of the component to be spaced from an anterior surface of the femur where the anterior flange terminates (i.e. the proximal end of the anterior flange 13).